Allow application/json uploads when fileinfo detects them as text/plain

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/54193

When the PHP fileinfo extension is active, it occasionally misidentifies uploaded .json files (especially those with simple string structures) as text/plain. Because application/json was not included in the fallback list of safe plain-text types in wp_check_filetype_and_ext(), these uploads were being falsely rejected.

This PR resolves the issue by adding application/json to the array of allowed types when the real MIME type is detected as text/plain. Since JSON is inherently a plain-text format, this is a secure, non-breaking fix that allows users to seamlessly upload JSON files, even on server environments with older libmagic definitions.

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